1. Field of the Invention
The embodiments disclosed relate generally to compressors and more particularly to poppet valves of hyper compressors with a converging-diverging flow passage and methods to reduce total pressure loss in these valves.
2. Description of the Related Art
Hyper compressors, those capable of producing gas pressure levels up to or above 3,000 bars, are widely used in industrial application, including, but not limited to, the production of low density polyethylene, or LDPE. The efficient performance of these compressors is controlled at least in part by suction and discharge automatic poppet valves. FIG. 1 illustrates a conventional poppet valve 10 in an opened position. As shown, the conventional poppet valve 10 includes a valve body 11 that contains therein a poppet, or poppet shutter, 12 configured to open and close the gas flow path in and out of a hyper compressor, a spring 14 configured to keep the poppet 12 in a closed position, and a poppet guide 16 that contains the poppet 12 and the spring 14. As shown, when the poppet shutter 12 is forced opened, flow passages 17 are formed from an inlet 18 to an outlet 20, the flow passage 17 being defined by the space formed between the poppet shutter 12 and the valve body 11 as well as by the space between the poppet guide 16 and the valve body 11. The poppet guide 16 of the conventional poppet valve 10 further includes a discharge opening 22 along an axis 24 of the poppet guide 16 connecting an inside chamber 26 of the poppet guide 16 to the flow passages 17 in a region of flow stagnation, the back pressure in the inside chamber 26 of the poppet guide 16 being defined at least in part by the static pressure around the axis 24 of the conventional poppet valve 10.
These poppet valves play an important role in the reliability of hyper compressors used in plants for the production of LDPE. The performance of such valves depends not only on selected material properties and a suitable design to withstand high operating gas pressures, but also on a proper dynamic behavior of the poppet shutter 12. The proper opening and closing of the valve are influenced by various design constraints related to several dynamic forces acting on the valve, including a drag force acting on the poppet shutter 12 and poppet guide 16 to open the valve, this drag force being generated by the interaction of the gas flow with the noted valve parts; a gas pressure force acting on the poppet guide 16 to close the conventional valve 10, this gas pressure force being generated by the flow back pressure acting on a back surface of the poppet guide 16; an inertia force associated with the mass of the poppet shutter 12; and a spring force generated by the spring 14 to close the valve, among others.
One example of the above-noted design constraints includes the requirement of a complete and steady opening of the shutter during the suction stroke of a piston of the hyper compressor. In this case, a reduced flow area may cause increased pressure losses and higher gas temperatures, leading to losses in compressor efficiency. Furthermore, an unstable motion of the shutter may also lead to a reduction in maintenance time between failures due mainly to the increase in the number of impacts between mobile and stationary parts. Another example of a design constraint relates to the requirement of shutter closure by the return spring before the piston motion reverses in order to avoid backflow. In addition, the motion of the poppet during a premature closure may be further accelerated by the gas drag force acting in the same direction as that of the spring force. Yet another example relates to the requirement to maintain impact velocities between mobile and stationary parts within allowable limits in order to prevent or minimize impact surface wear and the need to unnecessarily increasing the impact strength of valve components, thus increasing valve weight and cost. Finally, another example of a design constraint is the requirement for low sensitivity to a sticking phenomenon caused by, among other factors, the presence of lubricating oil and other contaminants in the gas causing sticking in various surfaces in contact with one another, resulting in impact velocity increases and valve closure delays.
Different factors, such as high gas temperatures, early wear, the presence of polymers, or loud noise, may be an indication of poor valve performance that may result in a reduction in the lifetime of the valve. Three-dimensional computational fluid dynamics (or CFD) has been extensively used to accurately simulate pressure losses, drag forces, pressure distributions, and flow coefficient at various valve-operating conditions. Based on these simulation studies and experimental results it is known that poppet motion can be correlated to critical performance factors and can be used to estimate valve life and that, in conventional valve configurations, the above-noted drag and pressure forces are not sufficient to either stably or fully opening the valve. Furthermore, in the flow passage of the conventional valve 10, while the valve is opening, an abrupt flow expansion is formed between the poppet shutter 12 and the inside surface of the valve body 11 at the region identified as element 28 in FIG. 1, thus causing a pressure loss in the flow that initially assists the valve to open, but that later, as opening proceeds, is detrimental to the dynamic performance of the valve because of the large pressure loss in that region and the associated generation of flow instabilities. This locally sustained pressure loss during the valve opening process together with simultaneous pressure increase generated by the back pressure in the inside chamber 26 of the poppet guide 16, as previously noted, therefore, lead to poor dynamic performance of the conventional poppet valve 10.
It would thus be desirable to develop a poppet valve for a hyper compressor with improved performance during the opening thereof while, at the same time, reducing the total pressure losses when the valve is fully opened, thus increasing compressor performance and reducing maintenance and downtime.